Self-locking threaded fasteners



Q United States Patent [1113,530,920

[72] Inventor Howard 1. Podell, 1,416,087 5/1922 Woodward", l5 l/2l(B)U)Larchmont, New York 1,626,863 5/1927 Nacey l5l/21(C)UX [21] Appl. No.731,416 I 1,660,455 2/1928 Plumb 151/21(C)UX [22] Filed May 23, 19682,177,004 1 10/1939 Purtelh... 151/22 [45] Patented Sept. 29, 19702,349,592 5/1944 Hosking 151/22 [73] Assignee USM Corporation, 2,886,0885/1959 Brancato 151/22 fgr r g i Primary Examiner-Ramon S. Britts acorp" o o w use) An0rneysW. Bigelow Hall, Richard A. Wise and Maurice R.

H I I Boiteau [54] SELF-LOCKING THREADED FASTENERS 9 Claims, 1] DrawingFigs. ABSTRACT: A self-locking threaded fastener such as a screw I inwhich locking action is obtained from at least one locking [52] U s c0/2 10/] 2 zone, parts of two adjacent turns leaning toward each other[51] [m m F16; 29/30 and including between them a groove of reducedthread angle. [50] rigid Search 51/22 The threads on the product areadvantageously formed by "I rolling with novel dies including a lockforming area in which 21B2lA&C 1on0 86A72/887 10/2 normal internalsupport for the flanks of developing thread ridges are removed duringthe latter part of the thread rolling [56] References Cited operationthereby causing the pressure applied to external UNlTED STATES PATENTSflanks to result in leaning of the locking turn portions toward 243,4936/1881 7 Bloom l5l/2I(C)UX each other.

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Patented Sept. 29, 1970 S et 1 of 3 v Inve r Howard P ge 53/ his Affnjpy SELF LOCKING THREADED FASTENERS This invention relates generally toself-locking threaded fastenersand more particularly to such fastenersin which a locking zone is formed in the thread itself without theintroduction' of added locking elements.

The widespread use of self-locking threaded fasteners in goodsmanufactured in large quantities is highly desirable because suchfasteners very often not only contribute to greater safety in the use ofthe goods but may also add to the reliability and durability of thegoods in areas of the product where safety in their use is not animportant factor. Although the use of self-locking threaded fasteners isgrowing rapidly, there have been a number of reasons which havecontributed to restricting their more extensive use. A major limitingfactor has been that of relatively high installed cost of conventionalself-locking threaded fasteners. In those fasteners which derive theirlocking action from a plastic locking element, several operations arerequired for mounting the locking element on the fastener. On the otherhand when the locking action is obtained by a special thread form in oneor both of two mating threaded members to produce aninterference fit,not only are added costs encountered in providing special tooling forthe manufacture of the special thread but also and more importantlyexpenditures are caused by the fact that such fasteners usually have asignificantly higher installation torque. Under the most adversetolerance conditions, seating torque may reach such a high level thatbolt heads are actually sheared off during installation. At any rate,heavier tools are required for a given screw size and production ratesare lowered thereby further adding to the installed cost.

Another important factor in limiting the more widespread use ofself-locking fasteners is the failure of conventional selflockingfasteners to satisfy the requirements of the environment-in which thefasteners must be used or of the normally expected conditions of use.For example, many of the conventional self-locking threaded fastenersare ineffective at temperatures exceeding 300F. In addition, fewconventional selflocking fasteners are completely reliable whensubjected to severe vibration for extended periods of time.

In addition to adverse environmental conditions of temperature andvibration another detrimental aspect of conventional self-lockingfasteners relates to the manner in which the locking action is produced.Generally in fasteners without added locking elements, the lockingaction is obtained by stressing one or both of the mating elementsbeyond their elastic limits. As a result such fasteners tend to showsubstantial loss of locking quality after limited use. When employedwith mating parts manufactured to standard commercial tolerances, suchfasteners often lose their locking characteristics almost completelywhen their first mating part is replaced by another.

lt is accordingly an object of the present invention to provide aself-locking threaded fastener in which the thread including a lockingzone may be produced economically on standard commercially availablethread forming machines employing standard tools requiring a minimum ofmodification.

Another object is to produce self-locking screws at speeds comparable tothose which would be achieved in processing standard non-locking screwsof comparable size and material.

Yet another object is to provide a locking thread structure adapted toretaining its locking action after repeated reuse and in spite of aninterchange of mating'parts.

in the achievement of the foregoing objects a feature of the inventionresides in the formation of a thread of a special locking form in apredetermined zone while threads of standard form may be produced alongthe remainder of the length of the fastener. In a screw according to apreferred form of the invention, for example, the thread is locallymodified in its form to produce a locking action for an extent rangingbetween a small fraction of a thread turn up to a half turn which willbe referred to as an elemental locking zone. It will be realized,however, that two elemental locking zones may be located end to end on ascrew and thus provide a composite locking zone extending 360 about theaxis of the screw.

2 The modification of the thread-in the locking zone while notcompletely uniform throughout its extent, is generally characterized byseveral variations from the standard thread. In a screw, the thread inthe locking zone is formed with a reduced thread angle, a shorteneddedendum, a broadened crest "width and an increased width across theflanks. As a result, when such a screw is engaged in a standardvthreaded opening or nut, there is a. controlled localized strain withinthe elastic limit of the material in the screw. The thread form in thelocking zone yields an interfering fit primarily in the flank area as itadapts .part are manufactured to broad commercial tolerances. and

' still obtain repeated locking action superior to that conventionallyattainable.

Another feature of the invention relates to the method by which screwsmay be processed so that the locking zone is formed at the same timethat the threads are rolled without requiring supplementary operations.in the practice of the method a screw blank of standard size is rolledbetween thread rolling dies which may be purchased as standard dies andreadily modified to produce the locking zone. It will first beunderstood that during a conventional thread rolling operation, materialof the screw blank is progressively forced and directed by'the serratedsurfaces of the dies from the valleys into the crest to form the thread.However, in practicing the present method at least one of the dies isprovided with a lock forming area in which the material of the screwblank is radially and axially compressed at points separated along theaxis of the screw while the intervening screw blank surfaces, which havealready been partly formed are left free of support and guidance. Thisis accomplished by eliminating from at least one of the dies the supportfor the intervening screw blank surfaces. This is conveniently achievedby removing a single ridge from the die for part of its length. Theresult of this relationship between screw blank and rolling die as theblank travels its last half revolution between the dies, is a preciseand predictable local advance of blank material into the thread groove.

Other features of the invention relate to the modified rolling diesthemselves, and to the extent of the support removed from the dies toproduce a novel screw having a plurality of locking zones atpredetermined relative locations in the finished product.

The foregoing objects and features and numerous advantages of thepresent invention will become more evident from the following detaileddescription of an illustrative embodiment of the invention together.with a description of the basic process by which the embodiment isrealized, taken in connection with the accompanying drawings in which:

FIG. 1 is a view in side elevation and on an enlarged scale, and with aportion broken away for clarity, of a screw including a locking zoneaccording to the present invention;

FIG. 2 is a plan view of a pair of rolling dies engaging a screw forforming threads including a locking zone in the shank of the screw;

FIG. 3 is a view in perspective of a pair of rolling dies one of whichhas been modified to produce a locking zone on a screw;

FIG. 4 is a view in cross section and on an enlarged scale of therelationship of a standard nut with a screw having a locking zoneaccording to the invention;

FIG. 5 is a view in perspective of a rolling die having a modified lockforming area;

HO. 6 is a fragmentary view on an enlarged scale showing the lockforming area in the die of F l0. 5. A

' FIGS. 7, 8 and 9 are progressive views partly in cross section and onan enlarged scale of a rolling die and screw showing in separatedrelationship and exaggerated for clarity the effect on a screw blankduring the last half revolution of rolling by a die modified to form alocking zone on the finished screw; 7

FIG. is a view showing in separated relationship a normal screw threadprofile greatly enlarged and matched with a standard nut profile; and

FIG. 11 is a view similar to FIG. 10 but showing screw threads of alocking zone according to the present invention engaging a standard nutprofile.

Prior to describing the embodiment of the invention as depicted in thedrawings it is considered desirable to explain certain departures fromnormal thread terminology which are adapted for convenience in thisspecification. Normally a single thread is defined as a single helicalridge of uniform section on the internal or external surface of acylinder, cone in the case of a tapered thread, left by forming acontinuous single helical groove in the appropriate cylindrical surface.Because the locking action in the locking zone according to the presentinvention is derived from localized variations in the form of the ridgeand of the groove and will more clearly appear in an axial section offasteners, it is considered more convenient and less cumbersome to speakof each turn as though it were a separate ridge or groove rather than asa continuation of an adjacent turn. Normal terminology such as crest,root, flank will be retained to refer to parts of a single turn orridge. Another departure from normal thread terminology necessitated bythe nature of the invention is the concept of pitch diameter which isnormally considered to be that of an imaginary cylinder passing throughthreads at such points as to make the width of ridge and groove equal.In this specifica tion, wherever locking action is described, atheoretical basic pitch diameter applicable to normal thread turns isestablished and those thread turns which provide the locking action areconsidered as abnormal at the theoretical pitch diameter.

Turning now to the drawings. particularly FIG. I. there is shown on anenlarged scale, a screw indicated generally at and including a head 22and a threaded shank 24. There are formed on the shank 24 a plurality ofthread turns of normal cross section comprising ridges 26 and grooves28. A locking zone comprises two ridges 30 and an intervening groove 32.The ridges 30 and the groove 32 comprise a single locking zone which maycover from a relatively few degrees to as much as a half turn about theaxis of the screw. A single screw may contain a single locking zone oralternatively a plurality of locking zones having different relativeplacements on the screw in order to suit the requirements of the matingpart which will normally be called simply a nut. A more completeunderstanding of the invention will be obtained, however, from aninitial consideration of the characteristics of the novel thread form ina single locking zone to be followed by a description of a preferredmethod of forming the single locking zone taken together with thetooling modifications necessary to produce it.

There is shown on a greatly enlarged scale in FIG. 10 the thread profileofa standard screw and in FIG. 11 a self-locking screw including thelocking zone consisting of the ridges 30 and the groove 32. The threadform in the locking zone of FIG. 11 may be directly compared not onlywith adjacent normal threads on the same screw but also with the profileof the thread of the standard screw. In FIG. 10 the standard threadprofile is designated generally as that of a screw 34 while the screwprofile of FIG. 11 may be considered as an enlargement ofa portion ofthe thread on the screw of FIG. 1. Outside the locking zone, the threadprofile of the self-locking screw includes normal ridges 26 generallycomparable in shape to ridges 36 ofthe screw 34 and grooves 28comparable to standard grooves 38. It will be appreciated that the twothreads whose profiles are depicted in FIGS. 10 and 11 were rolled withdifferent dies on different thread rolling machines and are related onlyin that both are enlargements of /420 threads rolled to a class 2tolerance.

In FIGS. 10 and I] the screws 34 and 24 are shown respectively withsimilar nuts 40 and 42 which have been drawn to the lead and threadangle of the screw 34 thereby assuming zero thread angle and leaddifference with a normal screw. Pitch lines 44 and 46 have been suppliedto the nuts 40 and 42 respectively and each of the pitch linesrepresents a mean pitch diameter of .2193 for a %20 nut manufactured toclass 2 tolerance. The major and minor diameters correspondingrespectively to root 48 and crests 50 of the nut 40 are respectivelyequal to .2500 and .2009 as are similarly roots 52 and crests 54 of thenut 42. The major diameter has been chosen as the minimum permissiblemajor diameter whereas the minor diameter is a mean figure for a class 2fit in %20 nuts. In the nut 40 the ridges and grooves are respectivelydesignated by reference characters 56 and 58. In the nut 42 of FIG. 12the ridges and grooves are respectively designated as 60 and 62. A pitchline 64 is shown on the threads of the screw depicted in FIG. 11. Alongpitch line 64 the normal or standard p ortion of the threads, the ridges26 and grooves 28 are of equal width. Similarly a pitch line 66 has beensupplied on the standard screw thread profile 34 of FIG. 10. Along thepitch line 66 all the ridges 36 and grooves 38 are of equal width.

Turning now to the angular orientation of the flanks of the ridges, itwill be realized that, in accordance with standard thread tolerancepractice, the thread angle which determines the relative orientations ofthe flanks on the ridges of the screw 34 and the nuts 40 and 42 is veryclose to the basic angle. Since these threads are of national form theangle closely approaches 60, any deviation from 60 together with anylead error in an actual thread being unspecified according to standardsbut rather absorbed in pitch diameter tolerances and allowances forfits. It is also seen that the thread angle between the ridge flanksdefining the normal grooves 28 of the selflocking screw 24 also closelyapproaches the standard 60 angle. However, the thread flanks definingthe locking groove 32 are relatively oriented at an angle somewhat lessthan 60 and consequently a locking action is obtained between the ridges30 at points near their crests contacting the flanks of the related nutcrest 54 as shown in FIG. 11.

It will be appreciated that the showings of FIGS. 10 and 11 do notrepresent conditions which are ever encountered during the engagement ofstandard nuts either with standard screws or with screws having lockingzones according to the present invention. Fundamentally these twofigures depict differences in clearances between standard matingthreaded parts shown in FIG. 10 in contrast to the clearances obtainedbetween a self-locking screw and standard nut as depicted in FIG. 11when the pitch line of nuts and screws in both figures are separated tothe same extent and the nut profiles are substantially identical. Inaddition it will further be appreciated that the angular orientation ofthe flanks defining the locking groove 32 is not uniform throughout thelocking zone but in fact continuously varies at different angularpositions about the axis of the screw. Starting at the entry into thelocking zone in one direction the angle of the locking groove 32 variesfrom a very slight gradually increasing deviation from the standard 60angle, increasing the deviation to produce a minimum thread anglecomparable to that of the groove 32 as shown in FIG. 11 and thereafterdecreasing to a lesser degree of deviation from normal but not returningcompletely to the normal thread angle in a gradual manner beforedropping off somewhat abruptly to the normal thread angle. Thus, in aright hand screw, the leading edge of the thread locking zone, thatwhich first engages the nut, is somewhat more abrupt than the trailingedge. This difference between leading and trailing ends of the lockingzone is a result of the fact that the leading end of the locking zone isin the process of being further deformed when the screw drops out fromengagement with the rolling dies at the end ofa rolling operation. Thisaspect of the invention will be more fully appreciated as will the othervariations from normal thread profile which produce the locking action,when the flow of screw blank material during the rolling action isexplained.

The thread rolling process and die configuration which will be explainedare also responsible for other deviations from standard thread form inthe locking zone. These other factors include slightly truncated crestson the locking ridges 30 resulting in a slightly broader and lowercrest, a root 68 of the locking groove 32, which is appreciably higheror at a greater radius than the roots of normal threads and a slightlygreater thickness of the locking ridges 30 along the pitch line 64.

In FIG. 4 a single locking zone encircled at 70 may be considered as theequivalent of the locking zone comprising the ridges 30 and the groove32 in FIG. 11 but will now be described in its locking effect inengagement with a nut 72 rather than by considering, as was done inrelation to FIGS. and 11, the comparison of normal and locking threadprofiles with their pitch lines in spaced relationship d withoutreference to the elastic deformation which takes place during theengagement of a self-locking screw according to the present inventionwith a standard mating part.

As depicted in FIG. 4 an upper locking flank 74 and a lower lockingflank 76 in the zone 70 have been elastically deformed generally toconform to the mating flanks of the nut 72. Under the condition depictedin FIG. 4 in which the screw including the locking zone 70 is free oftension, normal screw flanks 78 tend to be centralized and in spacedrelationship with nut flanks 80 in the axial plane and on the same sideas the locking zone 70. At the same time however the screw is shiftedsomewhat radially in the nut away from the locking zone 70 so that screwflanks 82 diametrically opposite the locking zone 70 are wedged intointimate contact with nut flanks 84. Thus locking action is obtainedfrom the forceful engagement of the flanks 74 and 76 as they areelastically deformed from their original, less than normal angularrelationship to conform to normal thread flanks 80 and from the tightengagement of the opposite screw flanks 82 with the nut flanks 84. Underconditions of screw tension, if it is assumed for example that anarticle of some thickness is tightly gripped between the nut 72 and ahead (not shown) on the screw including the locking zone 70, there isaresultant slight axial shifting of the nut on the screw as the lockingflank 74 and normal flanks 78 correspondingly oriented on the lowersurface of successive screw ridge turns are tightly engaged bycorresponding surfaces of the nut 72 while the screw flanks 78 on theupper surface of normal thread turns are spaced away from correspondingflanks of the nut. The flank 76 tends to return toward its unstressedstate, the crest terminating the flank 76 maintaining its contact withthe related nut flank and thereby contributing substantially to thelocking action. In order still further to improve the elastic clampingaction of the flanks 74 and 76 on the related flanks of the matingthread in the nut 72 locking screws according to the present inventionare, whenever feasible, of a greater hardness than the nut. Thus whenthenut is of relatively soft steel the screw may be of a heat treatedalloy steel or a case hardened mild steel of considerably greatersurface hardness than its mating thread. Similarly, as will readilyoccur to those skilled in the metallurgical art, non-ferrous screws maybe made harder than the mating threaded members by selecting differentanalyses, by heat treatment, or by work hardening, particularly workhardening inherent in the thread rolling operation.

In FIGS. 2 and 3, there is shown a screw comprising a threaded shank 86and a head 88 in the process of having its threads formed by rollingbetween a pair of dies comprising a stationary die 92 and a movable die94. The dies 92 and 94 are, except as will now be pointed out, standardin every respect, manufactured in large quantities and availablecommercially at low costs. Such commercial dies include work engagingsurfaces which are formed throughout their widths with alternatinggrooves and ridges 96 and 98 respectively spaced at intervals equal tothe pitch of the screw to be produced and with the grooves and ridgesoriented with respect to top surfaces 100 and 102 of the stationary andmovable die at an angle equal to the helix angle of the screw. Startingwith a commercially available pair of thread rolling dies, each having afull complement of grooves 96 and ridges 98, a lock forming area 104 isformed in the work engaging surface of one of the dies by removing afull ridge, generally by grinding since the dies are bought commerciallyin their hardened condition. It is convenient to think of the rollingdies as each having a leading end, that is an end which first engagesthe screw blank and a trailing end which is the last to engage thecompleted screw. Thus in FIGS. 2 and 3 the leading and trailing ends ofthe stationary die 92 are designated respectively by reference numerals106 and 108 while the leading and trailing end of the movable die 94 aredesignated by reference numerals 110 and 112 respectively. The lockforming area 104 in the die 92 is formed at a distance from the topsurface 100 along the trailing end of the die equal to the distancedesired between the underside of the screw head and the locking zone onthe screw.

The operation of the lock forming area 104 in producing a locking zonesuch as that comprising the ridges 30 and the groove 32 shown in FIG.11, will best be understood by considering the relationship of the lockforming area with the screw threads in the process of formation asdepicted in FIGS. 7 through 9 which shows in an exaggerated manner andin separated relationship for clarity successive relative positions ofthe stationary die 92 and a screw 114 having in an assumed axial planeupper and lower outer flanks 116 and 118 and upper and lower innerflanks 120 and 122. The screw and die conditions depicted in FIG. 9 maybe considered as that obtaining at the time when the screw 114 drops outof engagement with the rolling dies. FIG. 8 depicts therelationshipexisting between the screw 114 and the stationary die 92onehalf turn of the screw before reaching the position of FIG. 9,whereas FIG 7 illustrates the relative positions of screw and stationarydie a full turn before the position of FIG. 9 isreached. Accordingly,the plane of the screw depicted in FIG. 7 is the same as that depictedin FIG. 9 but with the depth of thread shown in FIG. 7 exaggeratedlyunformed or of shallower than normal depth, for clarity.

Keeping in mind-that the stationary die 92 and the movable die 94 arecontinuously being pressed into the body of the screw as it rolls fromthe position of FIG. 7 to that of FIG. 9 and that the threads areaccordingly being correspondingly deepened, it will be realized that asshown in FIG. 7 the outer flank 116 is not being worked and the threadroot terminating this flank, since it is not in engagement with a dieridge 98 remains shallower than the other root of the screw at the sametime and the flank 116 is somewhat thickened by not being worked. Inaddition since the inner flank 120 is fully engaged by a normal ridge 98of the die and the flank 116 is unsupported, die pressure is translatedinto displacement of the flank 116 upwardly.

When the screw 114 is in the position of FIG. 8 the flanks 116, 118, 120and 122 inclusive are in full engagement with a complete complement ofridges on the movable die 94 and the previous growth of the flank 116from not having been worked in the position of FIG. 7 together with theupward displacement from the pressure on the flank 120 while the flank116 was unsupported is partially corrected by the movable die. Theflanks 118 and 122 however, have maintained a normal or standardrelationship with the rolling die in both the positions of FIG. 7 andFIG. 8 and are of normal thickness and position. In rolling from theposition of FIG. 8 to that of FIG. 9, both of the outer flanks 116 and118 arrive in contact with ridges 98 which define the lock forming area104. However, the flanks 120 and 122 are unsupported so that thepressure on the outer flanks 116 and 118 is translated into adisplacement of the inner flanks 120 and 122 toward each other. Theflank 116 which was not worked in the positionof FIG. 7 and onlypartially reformed while the screw was in the position from FIG.

8, causes the flank 120 to be deflected from normal position to aslightly greater extent than the flank 122. Also, because no die ridgeis present to operate upon and support the flanks 120 and 122 in theposition of FIG. 9, the root betweenthese two flanks is slightlyshallower than the normal roots and the partial thread turns defined bythe flanks 118 and 122 and 120 and 116 tend to have a somewhat greaterthickness than the normal threads along their pitch line. This increasedthickness as well as the increased root radius between the flanks 120and 122 may be controlled accurately by die adjustment in the.-

thread rolling machine. The thread depth as measured radially from theroot of the locking groove to the crests of the locking ridges isgenerally greater than 90 percent of the depth of normal threads in thesame screw.

A number of characteristics of locking threads according to the presentinvention will now be apparent in view of the preferred method by whichthe locking zone is produced and of the tooling employed in itsproduction. It will first be appreciated that as the screw blankprogresses from the leading end 106 to the trailing end 108 of thestationary die, it typically completes four to six revolutions in acounterclockwise direction as seen in FlG. 2. From the start of theengagement of the screw blank between the dies, as the screw progressesand the thread on its shank is being formed to a greater depth,potential locking zones are continuously being formed and subsequentlybeing reformed into normal threads. As a result, the only locking zonein the finished screw is that at the predetermined level of the lockforming area in the last half revolution before the completed screwpasses the trailing end 108. The fact that the screw is rotating in acounterclockwise direction as viewed in FIG. 12 causes a slightly moreabrupt return to normal thread sections in the area of a radial plane onthe screw which coincides at the time of release with the trailing end108 of the stationary die 92. This factor is minimized however by thefact that standard commercial rolling dies are normally tapered awayfrom the flat work engaging surface for a distance equal toapproximately oneeighth inch at the end.

Another characteristic which has been mentioned is that the root betweenthe flanks 120 and 122 as shown in H6. 9 generally tends to be slightlyhigher than normal thread roots. Ample clearance is provided howeverbetween the higher than normal root and the minor diameter of nuts whichgenerally provide 75 percent or less of full thread engagement. Finallybecause the two locking ridges, the first defined by the flanks 116 and120 and the second by the flanks 122 and 118 are finally formed withoutconfinement by an intervening die ridge 98, the crests of these tworidges tend to be slightly lower and broader than normal. Neither theincreased breadth nor the slight truncation appreciably changes the fitof the screw 114 in a standard nut. The locking action is producedlargely by the localized pressure applied by the flanks 120 and 122which are relatively oriented at less than a normal thread angle, beingelastically deformed by engagement with normally oriented flanks of themating thread. The tendency of the normal mating thread to reform theangle between the flanks 120 and 122 is resisted by the elasticity ofthematerial and limited by the fit of the normal screw threads with themating part. Accordingly, upon disengagement from a mating thread theflanks 120 and 122 resume a position very close to their originalas-rolled position and therefore maintain their locking ability underthe most severe reusal tests. 7

It has been found not only that a locking zone may be formed in a screwduring the original rolling of the thread according to the presentinvention but also that screws having normal rolled threads may beprocessed with dies having a lock forming area such as that alreadydescribed to produce a screw having a lock forming zone characterized byselflocking characteristics very similar to those obtained in a screw inwhich the locking zone is formed at the same time that the normalthreads are originally rolled. Because a screw during a normal threadrolling operation is in very high compression along a line perpendicularto the work engaging surfaces of the die, the screw, while being rolled,assumes a slightly ovoid cross section which turns circular upon releasefrom the dies. It is for this reason that a normal or standard screw maybe rerolled to produce a screw having a locking zone, the differencebetween the minor axis of the stressed screw and the diameter of theunstressed screw essentially providing the material for the formation ofthe locking zone.

In the foregoing description of tools and processes for forming lockingzones on screws, the removal of a ridge extending the entire length ofone of the rolling dies has been suggested.

A useful alternative for practicing the invention is depicted in FIGS. 5and 6 in which a stationary thread rolling die 130 having a leading end132 and a trailing end 134 respectively comparable to the ends 106 and108 of the die 92, is modified to provide near the trailing end a lockforming area of limited longitudinal extent. The lock forming areaindicated at 136 is formed by grinding away part of a single ridge butonly for a length equal to one-half or even less of the pitchcircumference of the screw to be operated upon. It will be appreciatedfrom reviewing the description of successive relationships of the screw114 and the die 92 flow of the material of the screw 114, that a screwproduced by the die 130 is characterized in its locking zone by crestsmore nearly approaching normal height and form, a root of more normaldepth and a thickness at the pitch line of the threads more closelyapproaching standard values. However, the product of the die 130exhibits a locking thread groove of reduced thread angle at a pointcorresponding to the lock forming area 136 and such a screw has beenfound useful for applications not subject to extreme conditions ofvibration or forces tending to disengage the screw from its mating part.in addition, by reducing the length of the locking area 136 to adistance equal to something less than half the circumference of thescrew, for example one-quarter the circumference of the screw, anadditional element is introduced in the control of installation andremoval torque. As a result the basic configuration of the locking area136, varied in its length and coupled with the relative adjustment ofthe dies in the rolling machine, provides an effective control ofinstallation and withdrawal torque of the product to suit differentenvironments.

ln the foregoing descriptions of the locking zones 104 and 126 each hasbeen described as produced by the complete removal of a ridge on one ofthe rolling dies, in the case of the area 104 by removing the ridge forthe entire length of the die 92 and in the case of the area 136 byremoval of the entire ridge for a part of the length of the die 130. Itwill be appreciated however from the description of the lock formingaction as depicted in FIGS. 7 to 9 that the displacement of the flanksand 122 from the standard to the locking position is produced byremoving from between these two flanks the normal die support providedby a ridge 98 such as that which acts on the other threads of the screw.It is accordingly possible and within the contemplation of the presentinvention to employ a die having a lock forming area in which the wholeridge cross section has not been removed but rather in which the ridgehas been thinned along its flanks. This may readily be accomplishedstarting with a standard thread rolling die and grinding, for example.001", from each of the flanks of the ridge instead of removing theridge to produce the lock forming area. While this procedure is moretime consuming, requires greater accuracy and is accordingly moreexpensive, it does permit the exercise of an additional area of controlover the shape of the locking zone in the screw and of its lockingcharacteristics.

The product, tooling, and method according to the present invention havethus far been illustrated and discussed with specific reference to pairsof rolling dies in which a single die is modified by the provision of asingle lock forming area. From the foregoing description, however, itwill become clear to those of ordinary skill in the thread rolling artthat a wide variety of self-locking characteristics may be produced byobvious extensions of the present teaching as contained in theillustrative embodiments. For example, by providing lock forming areasin a single die at regular intervals it is possible to produce aself-locking screw having a plurality of spaced-apart locking zones.Self-locking action of such a screw may be'obtained in a standard nutplaced at any random position along the length of the screw simply byforming the individual locking zones closer together on the screw thanthe thickness of the nut. Other expedients which will become apparent tothose skilled in the art is that combinations of lock forming areas inboth dies either positioned so as to coact in the formation of a singlelocking zone of greater angular extent about the axis of the screw or toform separate locking zones spaced from one another along the length ofthe screw and staggered from side to side. Yet another expedient whichwill become obvious is that a lock forming area may be produced by theremoval of more than one adjacent ridges on a single rolling die oralternatively by grinding away the flanks of more than one adjacentridge. It is accordingly not intended that the scope of the inventionshould be limited to the specific illustrative embodiments hereindescribed.

I claim:

1. A self-locking male thread adapted for assembly with a matingthreaded member comprising a shank having a plurality of thread turnsspaced at their crests a normal axial distance apart and at least onelocking zone comprising axially aligned parts of two adjacent turnsleaning toward each other and defining between the crests of theadjacent turns an axial distance less than the normal axial distance.

2. A self-locking fastener adapted for assembly with a mating threadedmember comprising a body having a plurality of external thread turnsgenerally defined by flanks angularly oriented at a normal thread anglerelative to the adjacent flank of the adjacent turn, at least onelocking zone comprising axially aligned parts of two adjacent turnsleaning toward each other and defined by locking groove flanksrelatively oriented at an angle which is less than the normal threadangle, and said locking zone further having a flank oriented relative toa normal flank adjacent said locking zone forming an angle which isgreater than the normal thread angle.

3. A self-locking screw adapted for assembly into a threaded openingcomprising a shank having a plurality of external thread turns spaced attheir crests a uniform normal axial distance apart and defining a normalthread angle between the adjacent flanks thereof and at least onelocking zone comprising axially aligned parts of two adjacent turns atleast 90 percent of full thread depth, leaning toward each other anddefining between the crests of the adjacent turns an axial distance lessthan the normal axial distance.

4. A self-locking screw adapted for assembly into a threaded openingcomprising a shank having a plurality of external thread turns eachgenerally defined by flanks angularly oriented at a normal thread anglerelative to the adjacent flank of the adjacent turn and at least onelooking zone comprising axially aligned parts of two adjacent turnsleaning toward each other and defined by locking groove flanksrelatively oriented at an angle which is less than the normal threadangle.

5. A screw according to claim 4 further characterized in that thelocking groove flanks define angles of different sizes with a radialplane drawn between them.

6. A screw according to claim 5 further characterized in that the screwincludes a head and the angle between the radial plane and the flanknearer the head is smaller than that between the radial plane and theother locking groove flank.

7. A self-locking fastener adapted for assembly with a mating threadedmember comprising a body having a plurality of external thread turnsspaced at their crests a normal axial distance apart and at least onelocking zone comprising axially aligned parts of two adjacent turnsleaning toward each other and defining between the crests of theadjacent turns an axial distance less than the normal axial distance,and said locking zone further having a crest located relative to anadjacent normal crest defining an axial distance greater than the normalaxial distance between crests.

8. A self-locking fastener adapted for assembly with a mating threadedmember comprising a body having a plurality of external thread turnsspaced at their crests a normal axial distance apart and at least onelooking zone comprising axially aligned parts of two adjacent turnsleaning toward each other and defining between the crests of theadjacent turns an axial distance less than the normal axial distance andfurther characterized by one of the locking turns having a thread crestof truncated form relative to the crest of an adjacent normal thread.

9. A self-locking fastener adapted for assembly with a mating threadedmember comprising a body having a plurality of external thread turnsgenerally defined by flanks angularly oriented at a normal thread anglerelative to the adjacent flank of the adjacent turn and at least onelooking zone comprising axially aligned parts of two adjacent turnsleaning toward each other and defined by locking groove flanksrelatively oriented at an angle which is less than the normal threadangle and further characterized by one of the locking turns having athread crest of truncated form relative to the crest of an adjacentnormal thread.

